For years now, ultrasonic waves (i.e. high frequency acoustic waves) have been used to detect cracks, voids, porosity, and other internal discontinuities hidden in solid material such as metals, composites, plastics, and ceramics, as well as to measure thickness and analyze solid material properties. For instance, phased-array (PA) ultrasonic testing is an inspection technique which is known to be non-destructive, safe and, by now, well-established in the manufacturing, process, and service industries.
For instance, a typical PA inspection instrument generally comprises a probe and an inspection unit for operating the probe upon given parameters which can depend on the type of material of a test object to inspect, the geometry of the test object to inspect and/or the conditions in which inspection is to be performed. The typical probe includes an array of ultrasonic acoustic transducers which two main functions are, during use, to transmit time-shifted acoustic pulses in order to propagate a phased-array beam in the test object to inspect and to receive echo signals resulting from the propagation of the transmitted phased-array beams in the test object. Each acoustic pulse is time-shifted such that the individual acoustic pulses arrive in phase at a focal point after being refracted at an interface between the probe and the test object. Upon reception, the operating inspection unit is configured to display the received echo signals in a manner which allows a user to identify and localize flaws that may or may not exist in the inspected test object, for instance.
In use, however, the user may desire to inspect the test object using a phased-array beam having a propagation axis forming a given expected angle relative to the test object to inspect. To do so, the user inputs the expected angle in the PA inspection instrument which computes, using Snell's law of refraction, propagation instructions indicating how the acoustic transducer elements are to be collectively operated in order to propagate the requested phased-array beam. However, since the coupling efficiency of one acoustic pulse in the test object depends on its incident angle relative to the test object and on the acoustic velocity in the test object, not all the time-shifted acoustic pulses are evenly coupled in the test object, which can cause a bias (also referred to as “Snell's disparities”) between the requested phased-array beam and the phased-array beam truly propagated in the test object. More specifically, the phased-array beam actually propagated in the test object may form a biased angle relative to the test object rather than forming the expected angle. Such a bias can prevent the user from suitably positioning the flaws that may be present in the inspected test object, which can, in turn, cause time and cost inefficiencies.
To circumvent the challenge associated with the uneven coupling efficiency, U.S. Pat. No. 8,521,446 B2 to Zhang et al. proposes a method of calibrating the PA inspection instrument which compensates for the bias caused by the uneven coupling efficiency by measuring actual, true angles of the phased-array beams propagating in the test object to inspect using a calibration procedure. The proposed calibration procedure typically requires the user to perform high-precision, perhaps encoded, measurements using a reference block having fixed geometric and physical characteristics such as a precisely known acoustic velocity. Although appropriate in controlled conditions (e.g. in laboratories), this calibration procedure requires a degree of precision which is difficult to achieve on the field which may result in poor calibration of the phased-array inspection unit. There thus remains room for improvement.